The electronics industry has widely adopted Sn-3.0 Ag-0.5 Cu solder alloys for lead-free reflow soldering applications and tin-copper based alloys for wave soldering applications. In automated soldering or rework operations, users may work with Sn-Ag-Cu or Sn-Cu based alloys. One of the challenges with these types of lead-free alloys for automated / hand soldering operations, is that the life of the soldering iron tips will shorten drastically using lead-free solders with an increased cost of soldering iron tool maintenance/ tip replacement.

Development was done on a new lead-free low silver solder rework alloy (Sn-0.3Ag-0.7Cu-0.04Co) in comparison with a number of alternative lead-free alloys including Sn-0.3Ag-0.7Cu, Sn-0.7Cu and Sn-3.0Ag-0.5Cu and tin-lead Sn40Pb solder in soldering evaluations. Tests included solder alloy spread tests on copper, brass and nickel substrates. Soldering iron tip tests done with low silver cobalt containing alloy showed reduced erosion as compared to Sn-3.0Ag-0.5Cu solder alloy.

The cobalt in the lead-free solder wire was found to create barrier layers between the iron in the soldering tip and the solder, reducing solder tip erosion by as much as 50%. In addition, Sn-3.0Ag-0.5Cu surface mount soldered component test boards were reworked at the soldered chip component locations with Sn-0.3Ag-0.7Cu-0.04Co and Sn3Ag0.5Cu wire to simulate rework in manufacturing operations. Assessment also included pull tests of the soldered lead-frame component joints. The results of the tests are reported.

Conclusions

The evaluations indicated that the spreading of the developed Sn0.3Ag0.7Cu0.04Co solder wire on different substrates was equivalent to Sn3Ag0.5Cu solder wire. The Sn0.3Ag0.7Cu0.04Co showed low erosion on copper and iron compared with other lead-free solder alloys tested (Sn3Ag0.5Cu, Sn0.7Cu).

The results of the soldering iron tip erosion test showed the Sn0.3Ag0.7Cu0.04Co solder alloy had much lower solder iron tip erosion than lead-free Sn3Ag0.5Cu alloy and was similar or less than Sn40Pb tin-lead solder.

The intermetallic thickness growth for a 1st pass SnAgCu soldered chip component reworked with Sn0.3Ag0.7Cu0.04Co rework wire was lower than the chip components reworked with Sn3Ag0.5Cu rework wire at time zero and after thermal cycling and similar to 1st pass soldered SnAgCu soldered chip components.

Based on pull testing results of SnAgCu 1st pass soldered components and SnAgCu 1st pass soldered components reworked with Sn0.3Ag0.7Cu0.04Co rework wire at time zero and up to 2,000 ATC cycles there are similar pull test force results which indicate good reliability.

The results show promise for the use of the Sn0.3Ag0.7Cu0.04Co lead-free solder rework wire alloy in terms of good wetting, reduced solder iron tip erosion, reduced IMC growth and good reliability.

Initially Published in the IPC Proceedings

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